It honestly depends on what they've already accomplished by then.
If they've already pushed illegal immigration numbers down and repealed Obamacare (and replaced it with something better), I say they should steal an issue from the Democrats and work on criminal justice/sentencing reform for non-violent offenders. It's an issue that should have bipartisan support.
If they haven't accomplished both those things, then whichever they haven't already accomplished.
Also, maintaining/restoring the bipartisan support for Israel that has existed for over half a century should be of paramount importance. Appointing Mike Huckabee as ambassador to Israel could put an end to Democrats' support for Israel (as a party, not every individual member in Congress necessarily).
Trump could campaign against the current tax structure as well. The people who are most hurt by it are not the rich but people who are trying to be upwardly mobile, particularly those trying to make the jump from middle to upper class, but also those trying to enter the middle class.
I would agree with the issue of criminal justice reform.
Too many people are arrested for drug possession offenses and tattooed with a felony criminal record which never goes away without a pardon, which is not easy to obtain. This makes viable employment hard, and makes being shut out of the middle class a lifetime punishment for a youthful mistake.
There are some folks that ought to have a lifetime record, with folks being wary of them. But there are many convicted felons who are in that status because of old offenses that don't reflect at all the person they are today, and have been. I would hope that President Trump would do this for large numbers of Federal felons and encourage Governors of states to do the same for such offenders in states who no longer pose threats.